Minimize memory used by expiration manager

[ncabatoff]

After doing a restore of ~700K auth cert leases, 1.6.1:

Showing nodes accounting for 1768.55MB, 87.72% of 2016.18MB total
Dropped 184 nodes (cum <= 10.08MB)
Showing top 15 nodes out of 75
      flat  flat%   sum%        cum   cum%
  327.52MB 16.24% 16.24%   336.52MB 16.69%  encoding/json.(*decodeState).literalStore
  302.08MB 14.98% 31.23%   302.08MB 14.98%  reflect.mapassign
  248.07MB 12.30% 43.53%   248.07MB 12.30%  reflect.New
  241.07MB 11.96% 55.49%   241.07MB 11.96%  github.com/hashicorp/vault/vault.(*ExpirationManager).leaseTimesForExport (inline)
  102.52MB  5.08% 60.57%  1233.28MB 61.17%  github.com/hashicorp/vault/vault.decodeLeaseEntry
   89.56MB  4.44% 65.01%    97.06MB  4.81%  sync.(*Map).Store
   73.01MB  3.62% 68.64%    83.85MB  4.16%  github.com/hashicorp/vault/vendor/github.com/hashicorp/vault/sdk/logical.ScanView
   68.08MB  3.38% 72.01%    68.08MB  3.38%  encoding/json.(*Decoder).refill
   58.50MB  2.90% 74.91%    58.50MB  2.90%  reflect.makemap
   51.07MB  2.53% 77.45%    51.07MB  2.53%  github.com/hashicorp/vault/vault.(*AESGCMBarrier).decrypt
      51MB  2.53% 79.98%       55MB  2.73%  reflect.cvtBytesString
   47.07MB  2.33% 82.31%    83.07MB  4.12%  github.com/hashicorp/vault/vendor/go.etcd.io/bbolt.(*DB).View
      40MB  1.98% 84.30%   360.89MB 17.90%  github.com/hashicorp/vault/vault.(*ExpirationManager).updatePendingInternal
      36MB  1.79% 86.08%    41.20MB  2.04%  time.AfterFunc
      33MB  1.64% 87.72%       33MB  1.64%  reflect.unsafe_NewArray

With this branch:

Showing nodes accounting for 385.69MB, 97.30% of 396.41MB total
Dropped 143 nodes (cum <= 1.98MB)
Showing top 15 nodes out of 54
      flat  flat%   sum%        cum   cum%
  124.01MB 31.28% 31.28%   245.93MB 62.04%  github.com/hashicorp/vault/vault.(*ExpirationManager).updatePendingInternal
   86.01MB 21.70% 52.98%    86.01MB 21.70%  encoding/json.(*decodeState).literalStore
   81.07MB 20.45% 73.43%    94.07MB 23.73%  sync.(*Map).Store
      57MB 14.38% 87.81%    61.98MB 15.64%  time.AfterFunc
      13MB  3.28% 91.09%       13MB  3.28%  sync.newEntry (inline)
    9.50MB  2.40% 93.49%   375.58MB 94.75%  github.com/hashicorp/vault/vault.(*ExpirationManager).loadEntryInternal
    7.08MB  1.79% 95.27%     7.08MB  1.79%  github.com/beorn7/perks/quantile.newStream (inline)
    4.98MB  1.26% 96.53%     4.98MB  1.26%  time.startTimer
    2.04MB  0.51% 97.04%     2.04MB  0.51%  github.com/beorn7/perks/quantile.(*stream).merge
    0.50MB  0.13% 97.17%     7.58MB  1.91%  github.com/prometheus/client_golang/prometheus.newSummary
    0.50MB  0.13% 97.30%   376.08MB 94.87%  github.com/hashicorp/vault/vault.(*ExpirationManager).Restore.func2
         0     0% 97.30%    86.01MB 21.70%  encoding/json.(*Decoder).Decode
         0     0% 97.30%    86.01MB 21.70%  encoding/json.(*decodeState).object
         0     0% 97.30%    86.01MB 21.70%  encoding/json.(*decodeState).unmarshal
         0     0% 97.30%    86.01MB 21.70%  encoding/json.(*decodeState).value